Mutations Affecting the Sulphur Assimilation Pathway in Aspergillus nidulans: Their Effect on Sulphur Amino Acid Metabolism

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Abstract
Several sul-reg mutants of A. nidulans isolated as constitutive for arylsulfatase were studied with respect to the regulation of enzymes involved in cysteine and homocysteine synthesis and to the pool of S amino acids. All mutants examined showed a decreased concentration of glutathione as compared with the wild type, and all mutants, with one exception, had a decreased total pool of S amino acids. The mutants are probably leaky in the sulfate assimilation pathway. They show derepression of cysteine synthase, homocysteine synthase, cystathionine .beta.-synthase and .gamma.-cystathionase. In spite of having derepressed homocysteine synthase, the enzyme which constitutes an alternative pathway for homocysteine synthesis, the sul-reg mutations do not suppress lesions in genes required for the main homocysteine-synthesizing pathway. The derepression of homocysteine synthase is not in itself sufficient for physiological functioning of this enzyme, but seems to depend also on the effectiveness of cysteine synthesis and sulfide formation.

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